PV devices represent one of the major sources of environmentally clean and renewable energy. They are frequently used to convert optical energy into electrical energy. Typically, a PV device is made of one semiconducting material with p-doped and n-doped regions. The conversion efficiency of solar power into electricity of this device is limited to a maximum of about 37%, since photon energy in excess of the semiconductor's bandgap is wasted as heat. A fill commercialization of PV devices depends on technological advances that lead to higher efficiencies, lower cost, and stability of such devices. The cost of generating electricity may be significantly reduced by using solar modules constructed from inexpensive thin-film semiconductors such as copper indium di-selenide (CuInSe2 or CIS) or cadmium telluride (CdTe). Both materials have great promise, but certain difficulties have to be overcome before commercialization.
As shown in FIG. 1, the basic form of a CIS compound semiconductor thin-film solar cell (1) comprises of a multilayer structure superposed on a substrate (2) in the following order, a back electrode (3), a light absorbing layer (4), an interfacial buffer layer (5), a window layer (6) and an upper electrode (7). The substrate is commonly soda-lime glass, metal ribbon or polyimide sheet. The back electrode is commonly Mo metal
The light absorbing layer consists of a thin-film of a CIS p-type Cu-III-V12 Group chalcopyrite compound semiconductor. e.g. copper indium di-selenide (CIS). Partial substitutions of Ga for In (CIGS) and/or S for Se (CIGSS) are common practices used to adjust the bandgap of the absorber material for improved matching to the illumination.
CIS and similar light absorbing layers are commonly formed using various processing methods. These include; Physical Vapor Deposition (PVD) in which constituent elements are simultaneously, or sequentially, deposited onto a substrate. PVD methods include thermal evaporation. PVD processes typically employ expensive, high-vacuum equipment and the substrate sizes are constrained by the PVD technology and equipment.
A shortcoming of the conventional multi-source vacuum deposition methods is the difficulty in achieving compositional and structural homogeneity, both in profile and over large areas for device manufacturing. More specifically, device performance may be adversely impacted as a result of inhomogeneities, including semi-conducting properties, conversion efficiencies, reliability and manufacturing yields. Attempts to remove inhomogeneity through post-deposition processing are imperfect and can generate other detrimental effects.
Non-vacuum (NV) CIS film deposition has also been reported. NV methods include the formation of films from inks which contain dispersed nano-particles of constituent metals, or alloys, on substrates followed by post-deposition selenization (e.g. in H2 Se), and thermal treatments to effect the chemical reaction and/or consolidation of the films. Such post-processing is typically carried out at sub-liquidus temperatures of the material. An example of this approach is presented in U.S. Pat. No. 7,306,823 B2 by Sager et al.
Another reported NV approach employs the electrophoretic deposition of oxide particles from solution followed by additional post-deposition processing including chemical reduction, selenization and thermal processing. An example of this approach is presented in U.S. Pat. No. 6,127,202 by Kapur et al. NV techniques are considered for deposition on large area substrates, including flexible substrates. The possibility of using simple equipment and processes is seen as a viable route to low-cost manufacturing. The challenge for NV methods is to produce semiconductor films of sufficient quality to deliver good PV performance.
The aforementioned PVD and NV techniques for manufacturing CIS-based semiconductor thin-films for PV devices have not yet produced cost effective devices with conversion efficiencies that are sufficient for most practical applications.
In PV and other multi-layer devices, reactions and diffusion at film interfaces during deposition or during post-deposition processing may impact performance. For instance it has been shown that Na thermally diffuses from soda-lime substrates into CIS layers. In this instance the effect is found considered beneficial to the performance of the PV cell. Such effects are a natural consequence of the standard processing and are not independently controlled.
The highest performing CIGS solar cells to date have been reported by workers at the National Renewable Energy Laboratory (NREL), who claim 19.9% conversion efficiency under AM 1.5 illumination. This result is for small area devices produced by elemental co-evaporation onto soda-lime substrates. Larger area devices, manufactured by both vacuum and NV methods by various entities, on glass, metal ribbon or polymer substrates, more typically demonstrate area conversion efficiencies in the range of 8-12%. It is generally accepted that this lower performance is due to shortcomings in the device processing, including the light absorbing layer. For instance, there may be compositional non-uniformity, incomplete chemical and/or structural development and other defects across the area and profile and at the interfaces of the light absorbing layer.